Sterilization is the destruction of any virus, bacteria, fungus or other micro-organism, whether in a vegetative or in a dormant spore state. Conventional sterilization processes for medical instruments have involved high temperatures (such as steam and dry heat units) or toxic chemicals (such as ethylene oxide gas, EtO). Steam sterilization with an autoclave has been the time-honoured method of sterilization. It is fast and cost effective. However, the autoclave destroys heat-sensitive instruments. Thus, since more and more heat-sensitive instruments such as arthroscopes and endoscopes are used in medical treatment, other types of sterilization are needed, especially cold sterilization.
Ethylene oxide may be used to cold sterilize heat-sensitive instruments. However, It has now been deemed by national health and safety organizations to be carcinogenic and neurotoxic. It also poses flammability problems and is thus usually used in combination with chlorofluorocarbons (CFC's) which themselves are now undesirable. Further, sterilization with ethylene oxide takes 14 to 36 hours.
A more efficient, safer, and less expensive sterilization agent is ozone (O3). Ozone, especially humidified ozone, is a sterilizing gas. Ozone can easily be generated from oxygen, especially hospital grade oxygen. Oxygen is readily available in the hospital environment, usually from a wall or ceiling oxygen source, or, if mobility is required, from a portable “J” cylinder of oxygen. Ozone is widely used in industry as an oxidising agent to bleach paper pulp, treat drinking water, and sterilize sewage water and food products. The amounts (concentrations) of ozone required in the sterilization gas for water purification are low, generally less than 40 mg/l (milligram per liter). However, higher concentrations, combined with critical humidity levels, are required to make ozone an effective sterilant of micro-organisms. Those high concentrations of ozone gas have to be combined with critical levels of humidity. The sterilization efficiency of ozone increases rapidly with increased relative humidity. A high relative humidity is required for ozone to penetrate the protective shells of micro-organisms. The presence of water vapour will also accelerate ozone reactions with organic substances. Sufficient relative humidity further helps the penetration of sterilization packaging by ozone.
Sterilization with ozone is more efficient and quicker than with EtO and requires few changes in user habits. Moreover, ozone-based processes are compatible for use with current packaging, such as sterile pouches and rigid containers.
Ozone sterilization requires substantially no aeration or cooling down of sterilized instruments which can be used immediately following sterilization. This allows hospitals to reduce the cost of maintaining expensive medical device inventories. Ozone sterilization offers several other advantages. It produces no toxic waste, does not require the handling of dangerous gas cylinders, and poses no threat to the environment or the user's health. Stainless-steel instruments and heat-sensitive instruments can be treated simultaneously, which for some users will obviate the need for two separate sterilizers.
U.S. Pat. No. 3,719,017 discloses the use of a mixture of ozone gas with a very fine water mist in a sealed plastic bag container which contains an article to be sterilized. The method involves repeated evacuation and refilling of the plastic bag with a mixture of ozone gas and a very fine water mist. The air in the bag is exhausted and replaced with a pressurised mixture of ozone and water mist. Upon encountering the much lower pressure within the bag, the water particles from the pressurised mixture explode, forming a water mist. However, this system cannot generate a sufficiently high water vapour concentration to provide the high relative humidity required for thorough sterilization (at least 85% relative humidity).
U.S. Pat. No. 5,069,880 describes a device capable of generating a relative humidity of 85%. In the apparatus the ozone is bubbled through a water bath to increase the water content of the gas. Although ozone at 85% humidity can kill most micro-organisms, it does not meet the “worst case scenario” stipulated in North American standards. Moreover, the device is unable to generate humidity levels higher than 85%. In, addition, injecting ozone while humidifying the chamber increases the contact time of the ozone with the instruments to be sterilized, which may result in oxidation damage to the instruments.
A minimum relative humidity level of 90% (95%±5%) is required to meet North American standards set by agencies such as the Food and Drug Administration and Health Canada.
Water evaporates at 100° C. at atmospheric pressure (1013 mbar or 760 Torr). Thus, various prior patents (see Faddis et al., U.S. Pat. Nos. 5,266,275; 5,334,355; and 5,334,622) teach sterilization systems wherein water is heated to above the boiling point to evaporate the water for injection into the ozone-containing gas produced by an ozone generator. The steam is heated to 120° C. Thus, the vapour upon injection into the ozone-containing gas will have a temperature close to 100° C. However, since the decomposition of ozone increases exponentially with temperature in the range of 20 to 300° C., injecting the water vapour at a temperature of about 120° C. leads to premature ozone decomposition. As a result, the effective ozone concentration in the gas produced by the ozone generator is reduced, thereby requiring significantly increased treatment times and the generation of larger amounts of ozone gas for each sterilization cycle. Thus, a more efficient and effective sterilization apparatus is desired for the sterilization of ozone at a relative humidity of above at least 90%.
U.S. patent application Ser. No. 10/005,786 (filed on Nov. 8, 2001 which is a continuation-in-part application of U.S. patent application Ser. No. 09/310,695 which was filed on May 12, 1999 and is now abandoned), which is hereby incorporated by reference, addresses these problems by applying a vacuum pressure to lower the boiling point of water below the temperature inside the sterilization chamber. Thus the teachings of this application provide an effective sterilization process.
As taught in this prior application, it is preferred to repeat the sterilization cycle at least once to give greater assurance of effective sterilization. Thus, after loading the sterilization chamber with the articles to be sterilized (such as medical instruments), a sterilization cycle includes exposing the articles to the humidified ozone sterilant and then removing the sterilant. Repeating this cycle thus includes exposing the articles again to humidified ozone sterilant and removing the sterilant.
However, as mentioned above, in order to be sure of sterilization using ozone, the humidity should be at least 90% (95%±5%). Consistently achieving such high humidity levels has proved difficult. The sterilization chamber is in communication with a source of water vapour, for example, a water reservoir. As taught in U.S. patent application Ser. No. 10/005,786 mentioned above, a reduction in pressure will cause water in the reservoir to evaporate. However, this evaporation leads to cooling of the reservoir. Also, condensation of water vapour in the chamber tends to heat the chamber.
Any increase in the chamber temperature increases the quantity of water vapour required to reach the target humidity. Attempts to speed the process involve large thermal energy inputs, for example excessive heating of the water reservoir. This thermal energy eventually reaches the chamber and results in a temperature increase in the chamber which increases the quantity of water vapour needed for a given relative humidity. Thus achieving a high relative humidity with the consistency and accuracy needed to ensure complete sterilization is challenging.